1. Field of the Invention
The present invention relates generally to optical pickup devices and more particularly to a focusing error detecting apparatus for use with such an optical pickup device.
2. Description of the Prior Art
An optical pickup device or head is known to reproduce a signal from information recording pits on an optical recording medium such as an optical disc and so on. In this optical pickup head, a focusing servo is effected in order to accurately focus a beam spot of a laser beam emitted from the optical pickup head to the recording surface of an optical disc. To effect the above-mentioned focusing servo, a focusing error signal has to be detected. Various detecting systems have been proposed so far to detect the focusing error signal. FIGS. 1A to 1C respectively illustrate an optical system, a beam pattern on a light receiving element and a detecting signal utilized in a focusing error detecting system according to the so-called astigmatism system. This astigmatism system is widely used as one of the focusing error detecting systems and the details thereof are disclosed, for example, in U.S. Pat. No. 4,023,033. According to this astigmatism method, a laser beam emitted from a laser light source such as a semiconductor laser chip is irradiated on a recording surface of an optical disc such as an optical recording medium. The reflected laser beam from the optical disc is introduced into a beam splitter thereby to refract the optical path of this laser beam. Thereafter, the laser beam is traveled through a cylindrical lens 1 to be focused on a light receiving element such as a PIN diode of which the light receiving face is equally divided to provide four divided light-receiving portions in an astigmatism manner. As shown in FIG. 1A, such a beam spot as described above forms a circular beam spot 1b at the just focusing position, while such a beam spot forms an elliptic beam spot 1a which is long in the y axis direction at the position ahead of the focusing position; and it forms an elliptic beam spot 1c which is long in the x axis direction at the position behind the focusing position. Thus, if sum signals from the respective light receiving portions opposing to one another in the y- and x-axis directions in the four receiving portions are supplied to a calculating circuit 3 such as a differential amplifier as shown in FIG. 1B, in which a difference therebetween is calculated. When the beam spot diameter thereof lies at the just focusing 2b as shown in FIG. 1B, the focusing error signal from the calculating circuit 3 become zero. When the beam spot diameter is located at the point 2a ahead of the just focus 2b, a positive error signal is produced from the calculating circuit 3. When the beam spot diameter is located at the position 2c behind the just focus 2b, a negative error signal is produced from the calculating circuit 3. Thus, a focusing error signal having an S-shaped curve characteristic shown in FIG. 1C is produced from the calculating circuit 3. This conventional astigmatism method requires a cylindrical lens to focus a beam spot in an astigmatism manner. Further, an optical system must be aligned with high accuracy to precisely focus a reflected-back light from the optical disc at the center of the four light receiving faces.
To remove the above-mentioned defects, the same assignee of the present application has previously proposed a focusing error signal detecting apparatus in which the light receiving element is divided to provide three light receiving portions as is disclosed in Japanese Laid-Open Patent Gazette No. 62-197931. FIG. 2 illustrates an arrangement of such a focusing error signal detecting apparatus.
Referring to FIG. 2 forming a perspective view, there is provided an optical pickup device or head 5 in which first and second light receiving elements 7a and 7b such as PIN diodes for detecting a focusing error signal are provided on a major surface of a rectangular semiconductor substrate 6 made of silicon or the like at the position of the left-hand side half of the major surface in the drawing. The first and second light receiving elements 7a and 7b make two pairs of light receiving portions, each of which is divided to provide three portions to detect a laser beam. Further, a third monitoring light receiving element 8 formed of a PIN diode or the like is located at the right-hand half of the major surface of the semiconductor substrate 6. Between the first and third light receiving elements 7a and 8, a light emitting element 9 such as a semiconductor laser chip or the like is soldered directly to the semiconductor substrate 6 and a prism 10 of trapezoidal-shape in cross section is secured on the first and second light receiving elements 7a and 7b. Also, of a face 10b of the prism 10 contacting with the semiconductor substrate 6, other faces than the faces contacting with the first and second light receiving elements 7a and 7b and a face 10c of the prism 10 opposing the face 10b are all formed as the reflection faces.
With the above-mentioned arrangement, a laser beam 11a emitted from the active layer of the light emitting element 9 is reflected on a semi-transparent reflection face 10a of the prism 10 and is then irradiated, though not shown, through an objective lens on the optical disc as an incident beam 11b. A reflected beam of the incident beam 11b on the optical disc is traveled through the face 10a of the prism 10 and is made incident on the first light receiving element 7a of the first pair. The traveling light is reflected on the semi-transparent layer (not shown) provided on the first light receiving element 7a and between the prism 10 and the semiconductor substrate 6 and then reflected on the face 10c of the prism 10 to become incident on the second light receiving element 7b of the second pair, thereby to detect data and focusing error signal corresponding to the information pits on the optical disc. Reference numeral 11c designates a monitor laser beam emitted from the opposite active layer of the light emitting element 9.
FIG. 3 illustrates a plan view of the above-mentioned first and second light receiving elements 7a and 7b and a focusing error detecting circuit. The first and second light receiving elements 7a and 7b are respectively formed of three-divided light receiving faces 7a1, 7a2, 7a3; and 7b1, 7b2, 7b3. Of the thus three-divided light receiving portions, the central light receiving portions 7a3 and 7b3 are made narrow in width and the beam spots are respectively irradiated at the central positions of these light receiving portions 7a3 and 7b3. Detected signals derived from the right and left light receiving portions 7a1 and 7a2 of the first light receiving element 7a are supplied to an inverting input terminal of a first differential amplifier 3a and a detected signal derived from the central light receiving portion 7a3 is supplied to a non-inverting input terminal of the first differential amplifier 3a, thereby to derive a differential output A shown by a one-dot chain line in FIG. 4. In the second light receiving element 7b, detected outputs from the right and left light receiving portions 7b1 and 7b2 are supplied to an inverting input terminal of a second differential amplifier 3b and a detected output from the central light receiving element 7b3 is supplied to a non-inverting terminal of the second differential amplifier 3b, thereby to derive a differential output B shown by a broken line in FIG. 4. If the outputs A and B from the first and second differential amplifiers 3a and 3b are respectively supplied to a third differential amplifier, i.e., calculating circuit 3c at its non-inverting and inverting input terminals and a difference between the outputs A and B is thereby calculated, the third differential amplifier 3c produces a focusing error signal C shown by a solid line in FIG. 4. In FIG. 3, a solid line, a one-dot chain line and a dashed line on the first and second light receiving elements 7a and 7b illustrate the following conditions that the beam spot diameter of the laser beam incident on the optical disc is located at the just focus position 2b; it is located at the position 2a ahead of the just focus position 2b; and it is located at the position 2c behind the just focus position 2b, respectively.
With the above-mentioned optical pickup head structure, it is not necessary to provide a cylindrical lens which changes, when the focusing error signal is detected, the shape of the beam spot.
According to the focusing error detecting circuit shown in FIG. 3, unless the light emitting element 9 and the prism 10 are mounted on the semiconductor substrate 6 with excellent alignment accuracy in such a fashion that the reflected-back light from the optical disc irradiates the central portions of the central light receiving elements 7a3 and 7b3 of the first and second light receiving elements 7a and 7b, the focusing error detecting signal curve C (S-shaped curve) of FIG. 4 does not form an S-shaped curve which is symmetrical with respect to the just focus position D but it presents an asymmetrical property. Alternatively, the above-mentioned S-shaped curve C forms two or more of so-called zero-cross points with the result that the stable focusing servo cannot be effected.